[Bug rtl-optimization/68641] undefined variables implicitly considered to be zero

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68641

--- Comment #9 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 2 Dec 2015, Joost.VandeVondele at mat dot ethz.ch wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68641
> 
> Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |Joost.VandeVondele at mat dot ethz
>                    |                            |.ch
> 
> --- Comment #8 from Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> ---
> (In reply to Eric Botcazou from comment #3)
> > The whole reasoning looks fairly dubious to me, the optimizer is free to do
> > whatever it wants on undefined behavior and requests that the generated code
> > behaves identically at all optimization levels on it have little merit IMO.
> 
> Of course, I agree that the code has undefined behavior, and that 'all bets are
> off'. 
> 
> It just makes it more difficult for users to spot this undefined 
> behavior, we run our testsuite every night under valgrind, but can't 
> move from -O1 to -O0, since that would add a couple of hours to the 
> test. Admittedly, -fsanitize=memory would be a better solution, but it 
> is not available with gcc.

Though with the testcase you gave we warn at both -O0 and -O1:

> gfortran-5 t.f90 -c -Wall -O
t.f90:3:0:

   IF (i/=0) CALL link_error
 ^
Warning: ‘i’ is used uninitialized in this function [-Wuninitialized]
t.f90:4:0:

   IF (ASSOCIATED(foo)) CALL link_error
 ^
Warning: ‘foo’ is used uninitialized in this function [-Wuninitialized]